Thanks to NickF for these instructions:
Playing .iso DVD and Blu-Ray rips with JRiver Media Center
1. Go to Tools > Options > File types. Under "Data", click on "Disk Image Files" and at the bottom of the window, select your preferred Playback Method from the drop down list. Automatic can be used on the server and will mount the .iso into VCD (Virtual Clone Drive). If you use a different virtual drive, choose "Custom" and enter your program details. External Program (Custom) should be used on a client. Browse to the folder where your virtual drive .exe file is and select that file. For VCD, this will be Program Files\Elaborate Bytes\VirtualCloneDrive\VCDMount.exe.
2. Go to Tools > CD, DVD & BD and, under "Auto-Play" for the relevant disc types, select Play.
3. .iso files are currently treated as Data. Your library import needs to include these file types.
4. Go to Tools > Options > Library & Folders. Under "Auto-Import, Configure auto-import, select the folder or folders where your .iso files are stored, click on Edit and ensure that the Data box is checked.
5. Go to File > Library > Import and click on the Radar Button for "Run Auto-Import Now". This may take a while to complete.
6. In the left pane of the main window, click on Documents then Files. Your .iso files should show in the right pane. You may need to hover your pointer over the down arrow on the "Files" tab at the top and choose "List Style" then "Details". Right click on the title bar for the columns above the files and check Media Type and Media Sub-Type in the drop down list. This will cause these columns/Fields to show.
7. For your .iso files, Media Type will show as Data. Right click on Data and select Rename. Select Video from the drop down list. Right click on the blank entry in the Media Sub-Type column and select Rename and select Movie.
8. You may need to rename the file to the actual name of the movie.
9. In the left pane, select Video then Files. Scroll to find your file.
10. Right click on the file and select "Get Movie & TV Info" and select one of the data sources. This will populate many of the fields and insert Cover Art.
11. In order to get these movies to play smoothly, it is necessary to use madVR to change the frame rate. In madVR settings, under "devices" > "your display" > "display modes" enter the display modes required for the movie types you have, e.g. 1080p24, 1080p50, 1080p60, 1080p59.
12. You are now done. Double click the file and it should play. It will take a few seconds for the image to be mounted and for play to start.